Mark Twain, the celebrated humorist, delivered last evening, at Institute Hall, the first lecture of the "People's Course," to a good-sized audience. The subject announced was "Some Un-commonplace People I have Met"; but he opened his talk by saying that he had found it impossible to crowd any great number of people into his lecture, and therefore should lecture on Artemus Ward alone. Taking up Artemus, then, as a subject, he gave a sort of a sketch of his life as a fitting and convenient thread on which to hang the odd conceits, queer stories, and jokes which form the bulk of what he calls his lecture, generously attributing his own jokes to Artemus, and half the audience come away firmly convinced that they had read them in Artemus Ward's own works. The performance was an exceedingly pleasant one, Mr. Clemens having that air of quiet unconsciousness which seems inseparable from true humor, and his humor is as delicate and perfect as the perfume of some rare flower, whilst there was, we thought, a veiled sarcasm evident at times more just than generous to his pretended subject. We say again and, as Joaquin says, "we say it ever," that the talk was thoroughly enjoyable, but we fear that Mr. Clemens often veils his humor too closely for the easy recognition demanded by an audience.